Method for nitrogen-enrichment of molten steel covered with slag



Jan. 18, 1966 HAJIME NAKAMURA 3,230,075 METHOD FOR NITROGEN-ENRIGHMENT OF MOLTEN STEEL COVERED WITH SLAG Filed May 8, 1963 ABOUT 15T0 60 FIG. 2.

United States Patent 3,238,075 METHOD FOR NITROGEN-ENRICHMENT 0F MOLTEN STEEL COVERED WITH SLAG Haiime Nakamura, Tokyo, Japan, assignor to Ishikawajlrna-Harima Jukogyo Kabushiki Kaisha, Tokyo, Japan, a corporation of Japan Filed May 8, 1963, Ser. No. 278,901 Claims priority, application Japan, May 11, 1962, 37/ 18,577 1 Claim. (Cl. 75-59) It has become widely known in recent years that steels containing a properly dispersed metallic nitride, for example, aluminum nitride, are characterized by various desirable properties. For instance, the mechanical properties including tensile strength and, in particular, low temperature ductility of the nitride-containing steels are seen to be much improved over those of the steels of comparable chemical composition without the nitride component. Also such nitride containing steels are nonaging.

In the manufacture of such nitride-bearing steels, a series of steps comprising, first, enriching the steel while molten with nitrogen by suitable means, and then fixing the nitrogen in the steel by means of metallic elements which readily form hard nitride particles such as aluminum, beryllium, columbium, titanium, zirconium or vanadium, is generally adopted. In this case, the amount of nitrogen to be introduced in the molten steel and that of the metallic element to be added afterwards to the steel must be controlled effectively and desirable to the amount of metallic nitride to be contained ultimately in the steel product. However, the selection of the nitride-forming metallic element must be exercised with care, for not all of the known nitride-forming elements result in good mechanical properties for the steel.

As for the step of enriching the molten steel with nitrogen, a number of methods have been known; for instance, (I) a method in which the slag is made from potassium ferrous cyanate or potassium ferric cyanate (Imai and Ishizaki: Nippon Kinzoku Gakkaishi (The Journal of the Japan Institute of Metals), volume 16, Number 1, 1952), (II) a method in which the slag is made from calcium cyanide (Ca(CN) and silicate sand (T akadera: Tetsu To Hagane (The Journal of the Iron and Steel Institute of Japan), volume 27, Number 8, 1941), (III) a method in which the molten steel is stripped of its slag and made to contact with environmental air sufficiently long (ibid), (IV) a method in which nitrogen-containing ferrimanganese or water-free calcium nitrate is thrown into the melt (H. I. Wiester et al.: Stahl und Eisen, volume 77, 1957), (V) a method in which a certain airblast condition is observed in the pneumatic refining process executed by a Thomas Converter furnace (ibid), (VI) a method in which nitrogen gas or a gas mixture composed of nitrogen and an inert gas is blown into the melt under a particular pressure which satisfies the proper condition for nitrogen enrichment; and (VII) a method in which solid nitride that readily decomposes at an elevated temperature is blown into the melt together with nitrogen gas or inert gas or a gas mixture composed of nitrogen and inert gas under similar conditions with re spect to the carrier gas as described above (ibid).

It is said that any one of these methods mentioned above is effective for their respective case, but considering the present day situation of the steelmaking art in which the production of general service construction steel of so-called low carbon steel and low carbon, low alloy grade steel that occupy the most important portion of the market is being done by such large industrial steelmaking furnaces as large electric arc furnaces, oxygen over-blowing converter furnaces, and particularly, openhearth furnaces, many short-comings become apparent with these hitherto proposed methods. For example, the methods (I) and (II), mentioned above, of making the slag with special nitrogen-containing chemicals are not economical. The method (111) of making the molten steel to contact directly with air, can not readily be applied to a large furnace, even if the expense, time and human effort that must be put in the removal of the slag is justifiably disregarded, for the oxidation of steel occurs along with absorption of nitrogen. Although the alternative of the method (IV), the one that utilizes nitrogen-containing ferromanganese, appears to be the most realistic in practice, it has a shortcoming in that there is an unfavorable yield stability of the nitrogenenrichment. This is because when the slag layer that covers the molten metal is as thick as normally encountered in practice, the term-manganese suffers an unpredictably large loss due to oxidation while passing through the slag layer, and loses its nitrogen-enriching power before it ever gets to the surface of metal. The method (V), on the other hand, wherein the Thomas Converter is necessarily involved, is clearly of a limited value, for this kind of under-blowing converter is disappearing in steel mills.

In short, it may be concluded that as for the method for enriching molten steel under smelting within large steelmaking furnaces by nitrogen eifetcively, desirably, and at a scale that may appeal to the present day steelmaking industry, there is no other means available save the method either (VI) or (VII) of my invention. Namely, a method in which either nitrogen gas or a gas mixture comprising nitrogen and an inert gas is blown directly into the molten steel under such pressure that induces a proper stirring movement of the molten steel yet does not cause the molten metal to excessively splash out into the outer atmosphere, or a method in which decomposable solid nitride, such as calicum cyanamide, is blown directly into the molten steel on a stream of nitrogen or inert gas or a mixture thereof under similar condition and pressure as above. Because in any one of these methods, the nitrogenous agent, either in form of gas or gas and solid decomposable nitride, is blown directly into the molten bath, these methods will be referred to as the direct method hereinafter.

Although these direct methods of my invention are the sole practical methods of nitrogen enrichment that are applicable to present day large steelmaking furnaces such as open hearth furnaces, electric arc furnaces or over-blowing converters, they are not devoid of shortcomings. Namely, in order that the nitrogenous agent be blown directly into the melt while keeping the effective and stirring agitational movement of the molten steel by the carrier gas, the conduit pipe must be maintained straight throughout the blowing operation. Though this requirement may be met relatively easily by selecting a pipe with proper wall thickness or by putting thermoinsulative and thermoresistive protection, such a refractory clay, around the conduit pipe, and the material of such a property as to lit to the particular service condition may easily be found by those who are learned in the art, the success and the failure in obtaining the thermal protection of the conduit pipe is most directly related to the maintenance of the straightness of the pipe and thence to the nitrogen enrichment efiiciency. Thus, an expense and human effort is always demanded in preparing the more perfect conduit pipe, while the loss of the pipe in the metal bath by melting is also great.

The difiiculties mentioned above are particularly noted for an external heating furnace such as open-hearth furnace, where the maintenance of the straightness of the pipe is often an impossibility because of the very hot furnace atmosphere. The present invention is concerned with effective ways of nitrogen enrichment of molten steel which overcome the abovementioned difficulties. I discovered that when solid nitride in the form of particle or powder that readily decomposes at an elevated temperature is blown into the overlying slag and near the slag-metal interface on a stream of nitrogen gas or a gas that is inert both to the molten steel and the slag or a gas mixture composed thereof, the nitrogen enrichment of'the molten steel may be executed successfully at a stable nitrogen yield, without giving much attention to the straightness of the conduit pipe and markedly reducing the loss of the pipe, although the efficiency of the nitrogen enrichment is slightly lower than that common to the direct methods.

The theoretical ground on which this invention is based and the scope of this invention will be elucidated in the following, in which reference will be made to the draw- 1ngs:

FIGURE 1 is a drawing that shows schematically the case wherein a conduit pipe with thermal insulation is inserted into the slag layer; and FIGURE 2 is a drawing that shows schematically the case wherein a conduit pipe without thermal insulation is inserted into the slag layer. Thus, my invention relates to simple and effective methods of enriching, with nitrogen, molten steel in large industrial steelmaking furnaces such as open-hearth furnaces, electric arc furnaces or oxygen over-blowing converters, and wherein the melt is protected by a thick slag layer from direct contact with air. The method is characterized by blowing decomposable solid nitride such as calcium cyanamide into the slag layer at its bottom portion near the slag-metal interface through any suitable conduit pipe by nitrogen or a gas that is inert toboth the molten steel and the slag or a gas mixture composed thereof.

The feature that distinguishes this method from any other method of nitrogen-enrichment lies in the position of the nitrogen supply source with respect to the metal. Namely, whereas the nitrogen supply source in a normal method of nitrogen enrichment is always within the molten steel, in the method of my invention, a source of nitrogen supply which is made up mainly of the gaseous or nascent nitrogen liberated from the solid nitride blown in as described above, is positioned above the surface of the molten steel bath and in the slag layer yet held under such a condition where the direct contact with the outer atmosphere is effectively prevented by the overlying slag layer.

In other words, according to my invention, a conduit pipe whether thermally insulated or not, is introduced into the slag layer at an acute angle to the slag surface, and held so as to maintain its opening slightly above the slag-metal interface yet within the slag layer, namely, in the bottom portion of the slag layer. The position of the interface between the metal and the slag can easily be determined by the discontinuous increase of the resistance the operator feels as he inserts the conduit pipe through the slag. The angle that is formed between the slag, surface and the conduit pipe, though often dictated by the structure of the furnace, is preferably between about 15 degrees to 60 degrees. This arrangement is schematically shown in FIGURE 1. In the figure, 1 is the outer wall. of the furnace, 2 is a charging door, 3 is a conduit pipe, 4 represents thermal insulation around the pipe, 3, 5 is the slag, and 6 is the molten steel.

"The blowing of solid decomposable nitride such as cyanamide is commenced immediately after the conduit pipe is set to the proper position. A conduit pipe with s'ufiicient thermal insulation about it is invariably capable of maintaining the above-demonstrated relative position throughout the nitride blowing operation. However, when the conduit pipe is not insulated from heat, that portion of the pipe which is exposed to the hot atmosphere of the furnace starts softening as soon as it is introduced and bends downwards under its own weight and by the buoyancy exerted by the portion that is in the slag layer. The portion of the pipe which is within the slag layer, on the other hand, maintains itself straight due to the cooling effect of the gas stream that passes therethrough and to the insulation effect of the slag itself that is deposited and solidifies all over that portion of pipe. As the apparent specific gravity of the conduit pipe under such a condition as described above is far smaller than that of the molten steel and yet larger than that of the molten slag, the pipe takes a position in the bottom portion of the slag layer, lying there approxi rrnately parallel to the surface of the slag, without sinking into the molten metal bath. This situation is depicted schematically in FIGURE 2, in which the numerals correspond to those of the FIGURE 1.

In the situation described above, the solid nitride being fed to the bottom portion of the slag layer yet above the molten metal surface will undergo a decomposition reaction therein, which in the case of calcium cyanamide will be as follows:

Of the terms in the above equation, the Ca and C will interact with oxygen which is present in a form of FeO in molten steel and slag so as to drive it out of the steel, or to deoxidize the steel, as follows:

Of the terms in the above two equations, the CaO of the Equation 2 is absorbed into the slag, while the C0 of the Equation 3 gives rise to a strong agitation on the molten metal surface. The liberated nitrogen of the Equation 1 makes contact with the molten steel now under the agitation motion as described above, and since this state of motionis equivalent to a stirring action, the nitrogen gas induces within the molten steel bath nitrogen enrich: ment since the liberated nitrogen is readily absorbed in the molten steel. It is evident also that the reduction action of the Ca and C which is highly effective, though local, contributes greatly to this nitrogen enrichment reaction, for the fact that without an eifective deoxidation, no effective nitrogen enrichment is possible as acknowledged by all the authorities (for example, Takadera, op. cit.).

Thus, this principle of nitrogen enrichment is also applicable in the case where the raw calcium cyanamide which contains various impurities, principally free carbon, is used in place of the refined calcium cyanamide. The equations of the reaction for this raw cyanamide, which is often called agricultural grade calcium cyanamide, are much the same as the Equations 1, 2 and 3, as follows:

(CaONz+ GaO O) agricultural grade Care should be taken when the agricultural grade calcium cyanamide is used that the free carbon contained in it, if excessively present beyond what is necessary for the reaction of Equation 6, will work to carburize the steel.

Although it is evident that those nitrogen enrichment reactions described above may be obtained by any nitride that decomposes at an elevated temperature to liberate nitrogen other than the calcium cyanamide, the decomposition reaction of these kinds of nitrides is not accompanied by the creation of CO gas and resultant agitation action of the molten steel. It will be necessary to complement this lack of agitation action by, any suitable method, for instance, by blowing nitrogen gas or inert gas or mixture thereof directly into the molten bath, or by intentionally adding carbon to the nitride in an amount is listed in the Table 2 together with some information 'on 1 the acidity of the slag.

Table 2 .Ladle analysis, percent Slag, total Fe Steel Pre- Post- 0 Si Mn I P S Soluble N Nitrogen blow blow Al yield 12.20 I 9.80 0.20 l 0.20 0.32 0.034 0. 026 0. 045 0. 01s

used, for in such instance, the carbon liberated therefrom It is evident from the Table 2 that an effective nitromay not be sufiicient to satisfy the above requirement for gen-enrichment was successfullyperformed on the molthe agitation motion of the molten steel. 15 ten steel. The reason why the content of aluminum is It was found that a slag layer as thick as about 3 cm. unusually higher here than what is normally to be exis suiiicient to hold the nitrogen supply source produced pected of similar steels is that this element was added as heretofore disclosed from making contact with or intentionally so as to form aluminum nitride during a escaping out into the outer atmosphere. later stage.

The principle of present invention will be more fully EXAMPLE 2 understood in the following examples:

1 The same calcium cyanamide was blown in under the EXAMPLE same condition as the previous example, excepting the About 70 tons of steel was melted in a basic openquantity of the molten steel which was about 73vtons hearth furnace of nominal capacity of 60 tons. The r and the ratio of calcium cyanamide to molten steel which thickness of the slag layer after the completion of oxidizwas about 1kg;/ ton in this example. The higher calcium ing refining stage was about 28 cm. which was typical cyanamide to steel ratio was employed in order to obtain dark greyish oxidizing slag. A steel pipe about 4 m. higher nitrogen content. Factors such as the change in long was inserted into the slag layer without applying the slag, the nature, shape and size of the conduit pipe any thermal protection on it at an angle of about deand the extent of consumption of the pipe were all ahnost grees to the surface of the slag and held so as to leave 30 identical with those of the previous example. Table 3 about 1.5 in. extended outside the furnace. The portion presents the chemical composition of the steel and the of the steel pipe, about 1 m. long, which was exposed to slag thus obtained.

Table 3 .--Ladle analysis, percent Steel Slag. total Fe 0 Si M11 P S Soluble N Nitrogen I Al yield 10.93 0. 2s 0. 23 0. s1 0. 032 0.022 0.071 0. 021 15 the hot atmosphere within the furnace softened and bent It is evident here again that an effective and controlled about 20 seconds after the introduction of the pipe into nitrogen-enrichment was performed on the molten steel. the slag until three parts were clearly apparent, namely 4? The foregoing two examples illustrate indubitably that the straight portion of about 1.5 m. long that extends a the method of this invention is fully capable of controloutside the furnace, a softened curved portion of about lingly enriching the molten steel with nitrogen in a great 1 m. long that was heat softened by the high temperature quantity and at a stable yield. When this method is comfurnace atmosphere and a straight portion within the slag pared with the aforementioned direct method, also of layer of about 1.5 m. my invention, the following points become apparent.

Thereafter, calcium cyanamide of agricultural grade Namely, (l) at the similar carbon content level, the chiof the composition listed in the Table l was blown in at ciency of nitrogen-enrichment or the nitrogen yield, is a ratio of calcium cyanamide to molten steel of about about 15% in the present method, whereas it is about 0.8 kg. of calcium cyanamide per ton of the molten steel 17.5% or 21.5% in the direct method, if the overlying (0,8 kg./t0r Nitrogen gas was used as th carrier, It r5 slag is oxidizing or reductive, respectively; (2) the stawas ascertained by measurement that the conduit pipe 0 ility of the nitrogen yield is equally highly satisfactory remained almost unchanged in the initial state as deeither method; the p i l a t mu e eX- scribed above throughout the blowing operation, and that ercised in maintaining the straightness of the conduit pipe the consumption of the pipe was no more than about 25 in the direct method is not necessary in the present cm. at the pipe outlet. The pipe reclaimed after the method, thus making the operation much simpler and. operation was perfectly re-usable when the bent portion easier; (4) although the direct method is perfectly feasiwas straightened. ble even with conduit pipe without thermal protection for such furnace as electric arc furnace where the tem- Table chemwal 2 35; calcmm Cyanamlde perature of internal atmosphere is relatively low, and CaCN 55 the quantity of steel to be nitrogen-enriched small, hence z h an fthbl' 11hcao 33 t e ent1re me one e owing operation muc s ort C 12 er, but it is very diflicult to apply the direct method to open-hearth furnace Where the internal atmospheric tern- The reduction reaction shown by the Equations 5 and perature is very high and the quantity of steel to be 6 proceeded eifectively as the progress of blowing of calnitrogen-enriched much greater; (5) as the density, hence cium cyanamide, a fact that can be clearly seen in the the resistance to the carrier gas, is far smaller in the slag Table 2. layer than in the molten steel, the blowing can operate After the completion of calcium cyanamide blowing, under lower gas pressure in the present method than it the composition and the temperature of the molten steel is in the direct method; (6) whereas an additional and was adjusted, and the steel was tapped in a known manfavorable effect is gained in the direct method in which ner. The chemical composition of the steel thus obtained the blown-in gas induces stirring action in the molten metal which in turn promotes the coagulation and fast ascent of non-metallic inclusions, thus leaving the steel with less of those internal defects, similar effects and results are obtained in this method also due to the powerful agitation action which occurs at the molten steel bath surface.

Although only a basic open-hearth furnace has been shown in the actual examples given above, it should be immediately apparent to those who are skilled in the art that the principle of this invention is not limited to that particular furnace but is applicable to other large industrial steelmaking furnaces such as over-blowing converters or electric arc furnaces, and that the substance to be enriched with nitrogen is not limited to steel but includes any metal that is covered by slag.

I claim:

In the method of enriching with nitrogen, molten steel covered with a layer of slag, an improvement comprising blowing a nitrogen generating compound in a stream of gas into the bottom portion of the layer of slag but above the molten steel surface to establish a nitrogenization source at the interface between the layer of slag and the molten steel furnace, said nitrogen generating compound being selected from the group consisting of calcium cyanamide, nitride compounds which decompose at high temperatures and liberate nitrogen, and mixtures of the aforementioned substances of the group with free carbon, said gas being selected from the group consisting of nitrogen, gases which are inert to both the molten steel and the slag, and mixtures thereof, the nitrogen generating compound and the gas being blown into the slag layer by inserting a straight non-insulated conduit into the slag layer while allowing the conduit to soften under the heat of the slag layer and bend downwards under its own weight such that the conduit assumes a position in the bottom portion of the slag layer approximately parallel to the said interface, and passing the nitrogen generating compound and the gas through said conduit.

References Cited by the Examiner UNITED STATES PATENTS 2,537,103 1/1951 Tanczyn 75-51 2,918,365 12/1959 Kanamori et a1. 75-53 FOREIGN PATENTS 525,814 6/1956 Canada.

HYLAND BIZOT, Primary Examiner.

BENJAMIN HENKIN, Examiner. 

